Integrated circuits with recessed gate electrodes

ABSTRACT

Integrated circuits including MOSFETs with selectively recessed gate electrodes. Transistors having recessed gate electrodes with reduced capacitive coupling area to adjacent source and drain contact metallization are provided alongside transistors with gate electrodes that are non-recessed and have greater z-height. In embodiments, analog circuits employ transistors with gate electrodes of a given z-height while logic gates employ transistors with recessed gate electrodes of lesser z-height. In embodiments, subsets of substantially planar gate electrodes are selectively etched back to differentiate a height of the gate electrode based on a given transistor&#39;s application within a circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. patent application Ser. No. 16/020,722 filed Jun. 27, 2018, which is a Continuation of U.S. patent application Ser. No. 15/221,515 filed Jul. 27, 2016, now U.S. Pat. No. 10,020,232 issued Jul. 10, 2018, which is a Continuation of U.S. patent application Ser. No. 14/548,215 filed Nov. 19, 2014, now U.S. Pat. No. 9,418,898 issued Aug. 16, 2016, which is a Divisional of U.S. patent application Ser. No. 13/606,768 filed on Sep. 7, 2012 now U.S. Pat. No. 8,896,030 issued Nov. 25, 2014, the entire contents of which are hereby incorporated by reference herein.

TECHNICAL FIELD

Embodiments of the invention generally relate to integrated circuits (ICs), and more particularly relate to transistor gate electrode structures where some gate electrodes are recessed relative to other gate electrodes.

BACKGROUND

In modern complementary metal-oxide-semiconductor (CMOS) technology, parasitic transistor gate electrode capacitance is an ever more significant limiter of device performance. With device scaling, reductions in feature pitch continue to reduce distances between electrical nodes of a physical transistor. The introduction of high-k gate dielectrics may also adversely impact parasitic capacitances, for example where fabrication of the high-k gate dielectrics results in high-k films in regions other than at the interface between a transistor channel and the gate electrode. Furthermore, in addition to scaling and new materials introductions, structure geometries are now changing dramatically as the microelectronics industry now transitions from a planar to a non-planar field effect transistor (i.e., Tri-gate or FinFET). Pioneers of non-planar transistor technology are now developing second generation non-planar devices.

Techniques to reduce parasitic gate electrode capacitance are therefore advantageous. Furthermore, because gate electrodes for a given IC chip are typically all of a same z-height, unlike lithographically defined x and y dimensions of the gate electrode, transistor-level z-height control is also advantageous for selective tuning of gate electrode properties.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are illustrated by way of example, and not by way of limitation, and can be more fully understood with reference to the following detailed description when considered in connection with the figures in which:

FIG. 1A is a plan view of a layout of transistors in different circuits on a substrate, in accordance with an embodiment of the present invention;

FIG. 1B is a cross-sectional view of two transistors illustrated in FIG. 1A further illustrating one gate electrode recessed relative to another gate electrode, in accordance with an embodiment;

FIGS. 2A, 3A, 4A, 5A, 6A, 7A, and 8A are plan views of MOS transistors as particular fabrication operations are performed to selectively recess gate electrodes, in accordance with an embodiment;

FIGS. 2B, 3B, 4B, 5B, 6B, 7B, and 8B are cross-sectional views corresponding to the plan views in FIGS. 2A, 3A, 4A, 5A, 6A, 7A, and 8A, in accordance with embodiments;

FIGS. 9A and 9B are flow diagrams illustrating methods of selectively recessing gate electrodes, in accordance with embodiments;

FIG. 10 is an isometric view of a mobile computing platform employing an IC with transistors of recessed and non-recessed gate electrode z-heights, in accordance with an embodiment of the present invention; and

FIG. 11 is a functional block diagram of the mobile device illustrated in FIG. 10, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, numerous details are set forth, however, it will be apparent to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known methods and devices are shown in block diagram form, rather than in detail, to avoid obscuring the present invention. Reference throughout this specification to “an embodiment” or “in one embodiment” means that a particular feature, structure, function, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrase “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, functions, or characteristics may be combined in any suitable manner in one or more embodiments. For example, a first embodiment may be combined with a second embodiment anywhere the two embodiments are not specified to be mutually exclusive.

The terms “coupled” and “connected,” along with their derivatives, may be used herein to describe structural relationships between components. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” may be used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” my be used to indicated that two or more elements are in either direct or indirect (with other intervening elements between them) physical or electrical contact with each other, and/or that the two or more elements co-operate or interact with each other (e.g., as in a cause an effect relationship).

The terms “over,” “under,” “between,” and “on” as used herein refer to a relative position of one material layer or component with respect to other layers or components. For example, one layer disposed over or under another layer may be directly in contact with the other layer or may have one or more intervening layers. Moreover, one layer disposed between two layers may be directly in contact with the two layers or may have one or more intervening layers. In contrast, a first layer “on” a second layer is in direct contact with that second layer. Similarly, unless explicitly stated otherwise, one feature disposed between two features may be in direct contact with the adjacent features or may have one or more intervening features.

In embodiments, integrated circuits (ICs) including MOSFETs have selectively recessed transistor gate electrodes. In other words, a transistor with a gate electrode of lesser z-height (i.e., a recessed gate electrode) is monolithically integrated with a transistor having a gate electrode of greater z-height (i.e., non-recessed gate electrodes). As such, in addition to x and y dimensions of a gate electrode being definable at a transistor-level, so too is the gate electrode z-dimension. Transistors with gate electrodes of lesser z-height benefit from reduced parasitic capacitive coupling area, for example, to adjacent source/drain diffusion contact metallization. Parasitic gate capacitance can be most simply modeled as C=εA/d with A being proportional to z-height of a gate electrode, d being the spacing between the gate electrode and adjacent source/drain contact metallization, and ε being an effective dielectric constant of material disposed between the electrode and contact metallization. As such, a parasitic capacitive area associated with a gate electrode may be modulated through fabrication, along with other structural attributes of the gate electrode, such as gate length (L_(g)) and gate width, based on a given transistor's function within a circuit. Similarly, other gate electrode properties beyond parasitic capacitive area, such as, but not limited to, work function, may be tuned through a modulation of z-height. In embodiments, transistor gate electrode z-heights are differentiated based on individual circuit demands. For example, in circuits where maximum consistency of gate electrode dimension is advantageous for analog transistor matching, or where retention of a greater amount of one or more gate materials enables advantageous tuning of transistor gate electrode work functions, transistors of a greater gate electrode z-height may be employed alongside circuits employing transistors with a recessed gate electrode where minimum parasitic gate capacitance is advantageous.

In one exemplary embodiment, one or more logic gate is recessed from a z-height associated with one or more analog gate. In other words, a greater gate electrode z-height may be provided for an analog gate than a logic gate. In certain such embodiments, all logic gates of a particular IC are recessed from a nominal z-height of all analog gates in the IC. In other embodiments, gate electrode z-height, and the associated parasitic gate electrode capacitance, is differentiated on another basis where only a subset of logic transistors have recessed gate electrodes, or where one or more analog transistors have recessed gates. For example, gate electrode z-height may be modulated between transistors on the basis of conductivity type where NMOS and PMOS transistors have different gate electrode z-heights. In still other embodiments, gate electrode z-height may be varied between two recessed gates. For example, a first transistor in a logic circuit may be recessed relative to reference z-height (e.g., that of an analog gate) by a first amount, such as 25%, while a second transistor in the logic circuit may be recessed by a second amount, such as 45%. While gate electrode z-height differentiation is most thoroughly described herein in the context of logic and analog gates, it will be appreciated that the exemplary structures are equally applicable for differentiating gate electrode z-heights based on any circuit design criteria, or transistor function.

FIG. 1A is a plan view of a layout of certain transistors in a microelectronic device 100, such as an IC, formed on a substrate 105. The microelectronic device 100 includes a circuit 101 and a circuit 202. Generally, the circuit 101 represents a portion of any circuit employing a MOSFET logic gate electrode 120 (i.e., logic circuitry). In the illustrated embodiment, the circuit 101 is an inverter (i.e., NOT gate). In one such embodiment, the circuit 101 is a portion of a static random access memory (SRAM) circuit, for example part of an SRAM cache memory of a microprocessor. The circuit 202 generally represents a portion of any circuit employing MOSFET analog gate electrode 220 (i.e., analog circuitry). For example, the circuit 202 may be a portion of an amplifier circuit, driving a load in response to a control voltage on the gate electrode, as for an I/O circuit of the microelectronic device 100, as part of a thermal sensing circuit, or part of a band gap reference circuit, etc. As illustrated, the MOSFET logic gate electrode 120 has a gate length, L_(g.1) and the MOSFET analog gate electrode 220 has a gate length, L_(g.2). Generally, analog gates are of a greater length than logic gates. In the exemplary embodiment, L_(g.2) is at least 50% larger than L_(g.1). Although the circuits 101, 202 are separated only by an isolation region 150 of minimum dimension, the circuits employing transistor with differentiated gate electrode heights may also be separated by much greater distances, for example where the circuit 101 is located in a core of a microprocessor and the circuit 202 is located in a periphery of the microprocessor core.

As further illustrated in FIG. 1A, the circuit 101 includes a first MOS transistor utilizing a non-planar semiconductor “fin” 110A disposed over a first portion of the substrate 105 while the circuit 202 includes a second MOS transistor utilizing a non-planar semiconductor fin 210A. While the exemplary embodiments utilize non-planar MOSFETs, or “finFETS,” the skilled artisan may also apply the teachings herein to a planar MOS transistor. In one embodiment illustrated by FIG. 1A, gate electrodes of different heights are employed in the different circuits 101 and 202. For example, a transistor of circuit 101 includes a logic gate electrode 120 disposed over the fin 110A that is “recessed from” or “shorter than” the “non-recessed” analog gate electrode 220 disposed over the fin 210A found in the transistor of circuit 202.

FIG. 1B is a cross-sectional view of the two transistors 110A and 210A along the dashed A-A′ line of FIG. 1A and further illustrates a z-height difference between the logic gate electrode 120 and analog gate electrode 220. As shown in FIG. 1B, the logic gate electrode 120 is disposed over a semiconductor channel region 140 of the fin 110A while the analog gate electrode 220 is disposed over a semiconductor channel region 240. Both the gate electrodes 120 and 220 have bottom electrode surfaces proximate to the semiconductor channel regions 140, 240, respectively, with a gate dielectric 165 disposed there between. In the exemplary embodiment, bottom electrode surfaces of the gate electrodes 120 and 220 are at a same reference plane R-R′, however bottom gate surfaces may also be different between gate electrodes of different z-height. The analog gate electrode 220 has a top surface portion 261A at a z-height H₁ from the bottom gate electrode surface disposed at R-R′ while the logic gate electrode 120 has a top surface portion 161A at a gate electrode z-height H₅ from the bottom gate electrode surface disposed at R-R′. As shown, at least a portion of the top electrode surface 161A is recessed below at least a portion of the top electrode surface 261A such that the gate electrode z-height H₅ is different than the gate electrode z-height H₁ by the recess amount H_(recess). Although the gate electrodes 120 and 220 are depicted as having perfectly flat, or level top surfaces, real devices can be expected to have at least some variation in z-height at different points within a given gate electrode. As such, H₁, H₅, and similar quantities, may be considered averages taken over a same area over a semiconductor channel region and between source-drain contact metallization.

Generally, the recess amount H_(recess) is more than the z-height variation to be found in the gate electrode height H₁ such that a recessed gate electrode with z-height H₅ is statistically different from a population of gate electrodes characterized by a nominal z-height H₁. A population of transistors with recessed gate electrodes characterized as having a z-height H₅, along with a population of gate electrodes characterized by a nominal z-height H₁, would form a bimodal distribution of gate electrode z-heights. In embodiments where good z-height control is achieved for the analog gate electrode 220 (i.e., deviation about H₁ is small), H_(recess) may be as little as 5% of H₁. Noting a greater H_(recess) corresponds to greater reduction in parasitic capacitance associated with the logic gate electrode 120, H_(recess) is at least 15% of H₁ in more favorable embodiments, at least 25% of H₁ in highly advantageous embodiments, and at least 30-50% of H₁ (i.e., H₅ is 50%-70% H₁) in exemplary embodiments. Although a practical upper bound on H_(recess) may vary considerably as a function of implementation and process tolerances, z-height may differ between electrodes by even 80%.

As illustrated in FIGS. 1A and 1B, the semiconductor fin 110A further includes a first source/drain region 130A and a second source/drain region 130B disposed on either side of the logic gate electrode 120 while the semiconductor fin 210A further includes a first source/drain region 230A and a second source/drain region 230B. As further illustrated in FIG. 1A, the exemplary microelectronic device 100 replicates each of the semiconductor fins 110A, 115A, 210A, and 215A as fins 110N, 115N, 210N, and 215N, respectively, having source/drain regions (e.g., 130C, 130D, 230C, 230D) that also couple to source/drain contacts (e.g., 131A, 131B, 231A, 231B). Any number of such fins may share a same gate electrode.

The source/drain regions (e.g., 130A, 130B, 230A, 230B) may be doped regions of the semiconductor fin 110A and may include regrown or epitaxially deposited semiconductor regions such that the z-heights of the source/drain regions exceeds that of the channel regions 140, 240, as depicted. In one exemplary embodiment where the circuit 101 is a CMOS inverter the circuit 101 includes a transistor of a first conductivity type (e.g., N-type) and a transistor of a second, complementary, conductivity type (e.g., P-type). For example, referring to FIG. 1A, where a first transistor employing the semiconductor fin 110A has source/drain regions 130A, 130B of a first conductivity type (e.g., N-type); a second transistor sharing the logic gate electrode 120 employing a semiconductor fin 115A has source/drain regions 135A, 135B of a second conductivity type (e.g., P-type). Each of these source/drain regions are further coupled to source/drain contacts 131A, 131B, 136A, 136B, drawn in dashed line for the sake of clear illustration. Notably, although in the exemplary CMOS inverter embodiment the logic gate electrode 120 has the approximately the same z-height (e.g., H₅) over both the semiconductor fins 110A and 115A, the techniques described herein also enable gate electrode z-height differentiation even between two transistors having a same gate electrode. For example, the height of the logic gate electrode 120 may vary from the z-height H₅ proximate to the semiconductor fin 110A to the z-height H₁ proximate to the semiconductor fin 115A. Furthermore, although not depicted, it should be appreciated that the gate electrode z-height differentiation illustrated in FIG. 1B between a logic transistor and an analog transistor may also be extended to three or more different gate electrode z-heights. As one example, for an SRAM embodiment employing the circuit 101, a pull-down transistor may have a first recessed gate electrode (e.g., recessed 45% relative to an analog gate of the IC) while a pass transistor has a second recessed gate electrode (e.g., recessed 25% relative to an analog gate of the IC).

In embodiments, an entire width of a gate electrode is recessed. For example, referring to FIG. 1A, where the logic gate electrode 120 makes a stripe, the entire stripe is of a same z-height (e.g., H₅ from FIG. 1B). In alternate embodiments, where a gate electrode is recessed only along portions of its width, the gate electrode has a reduced z-height along a width of the gate extending adjacent to a source/drain contact. For example, as also illustrated in FIG. 1A, the microelectronic device 100 may include any number of non-planar transistors electrically coupled together in parallel for a greater current carrying channel width and embodiments of the present invention are not limited in this respect. For the exemplary microelectronic device 100, each of the semiconductor fins 110A, 115A, and 210A are replicated as fins 110N, 115N, and 210N, respectively, having source/drain regions that also couple to source/drain contact metallization. In the exemplary embodiment where a single contact metallization 131A spans multiple fins 110A and 110N, the logic gate electrode 120 is recessed along a width adjacent to the contact metallization 131A. The logic gate electrode 120 may then be similarly recessed along the width adjacent to 135A, with a full z-height (e.g., H₁) in the intervening span of the logic gate electrode 120, for example where the logic gate electrode 120 may be contacted by an upper level metal.

In embodiments, a gate electrode with a reduced z-height has a top gate electrode surface that is recessed below a top surface of the contact metallization by an amount greater than is the top electrode surface of a gate electrode having a full z-height. For example, as illustrated in FIG. 1B, the logic gate electrode top surface 161A is recessed from the contact metallization top surfaces 151A, 151B while the analog gate electrode top surface 261A is not recessed from the contact metallization top surfaces 251A, 251B, and may actually be proud of the top surfaces 251A, 251B, as depicted. In further embodiments, a gate electrode with a reduced z-height has a top gate electrode surface that is recessed below a top surface of an adjacent spacer dielectric. For example, as also illustrated in FIG. 1B, the logic gate electrode top surface 161A is recessed from a top surface of the dielectric spacer 155 while the analog gate electrode top surface 261A is not recessed from the adjacent dielectric spacer 155.

In embodiments where gate electrodes include a bulk material and a work function material (i.e., a gate electrode stack), at least the bulk material of a recessed gate electrode has a reduced z-height relative to bulk material of a non-recessed gate electrode. In the exemplary embodiment illustrated in FIG. 1B, the gate electrodes 120, 220 have work function materials 120A, 220A respectively disposed between the bulk materials 120B, 220B and the gate dielectric 165. In the depicted embodiment where the gate electrodes are formed by filling a trench, as in a gate replacement process, the work function materials 120A, 220A are disposed along opposite sidewalls of the bulk materials 120B, 220B with both the bulk materials 120B, 220B and the work function materials 120A, 120B present at portions of the first and second top electrode surfaces 161A, 261A, respectively. Generally, the work function materials 120A, 120B may be any known in the art. Notably, the work function material 120A may also be different than the work function material 220A, for example where different work functions are desired for transistors of different conductivity type. Similarly, the bulk materials 120B, 220B may be any known in the art, such as, but not limited to polysilicon or one or metals (e.g., tungsten (W), copper (Cu), aluminum (Al), their alloys, etc.). For alternative embodiments where a gate electrode is a homogenous material, the entire gate electrode is represented by the bulk material illustrated in the more complex gate electrode stack.

As illustrated in FIG. 1B, both the bulk material 120B and the work function material 120A in the logic gate electrode 120 are recessed, to z-heights H₅ and H₆, respectively. Depending on the embodiment, H₅ and H₆ may be approximately equal, H₆ may be significantly less than H₅, as depicted, or H₆ may be significantly larger than H₅, (e.g., where work function material stringers are left as fabrication artifacts along the adjacent gate dielectric 165 and/or spacer dielectric 155 separating the logic gate electrode 120 from the adjacent source/drain contacts 131A, 131B). In advantageous embodiments, both H₅ and H₆ are recessed by H_(recess) that is at least 5%, and more advantageously at least 25% and most advantageously at least 30-50% the z-height H₁. Reducing z-height of both bulk and work function materials may have a greater impact on parasitic capacitance, although depending on the conductivity of the work function material 120A, recessing of the bulk material 120B alone may provide a notable reduction in parasitic gate capacitance, or favorable tuning of work function. Conversely, having the analog gate 220 at a z-height of H₁ may advantageously provide a desired amount of one or more work function material 220A and/or bulk material 220B desired for analog circuit performance (e.g., to achieve a particular work function for the analog gate 220 or achieve a level of matching between two analog gates). While the illustrative embodiment depicts gate electrode structure indicative of a gate replacement fabrication technique, embodiments of the present invention are also applicable to “gate first” fabrication techniques where a gate electrode top surface typically entails only the bulk material with the work function material and/or the gate dielectric 165 absent from the sidewalls of the bulk gate material. Thus, in both replacement-gate and gate-first embodiments, at least the bulk material of a recessed gate electrode has a reduced z-height.

Depending on the technique employed to selectively differentiate gate electrode z-heights between transistors, the amount of recess between two gate electrodes may further differ between bulk materials and work function materials as a function of a difference in z-height between the bulk material and the work function metal present in a gate electrode having the greater z-height. For example, as illustrated for the analog gate 220 in FIG. 1B, the work function material forms a sidewall having a top surface 261B at z-height H₂ from the bottom gate electrode surface on the R-R′ plane. In the exemplary embodiment where H₂ is less than H₁ associated with the bulk material, the work function metal of the analog gate 220 is itself recessed relative to the bulk material such that the top surface portion 161A of the logic gate electrode 120 is recessed from the bulk material top surface 261A more than it is from the work function material top surface 261B.

With structural features associated with embodiments of the present invention described, techniques for selectively recessing gate electrode in accordance with embodiments of the present invention are now described. FIGS. 2A, 3A, 4A, 5A, 6A, 7A, and 8A are plan views of transistors as particular fabrication operations are performed to selectively recess gate electrodes, in accordance with an embodiment. Such techniques may be utilized to arrive at the structures of FIGS. 1A, 1B. FIGS. 2B, 3B, 4B, 5B, 6B, 7B, and 8B are cross-sectional views corresponding to the plan views in FIGS. 2A, 3A, 4A, 5A, 6A, 7A, and 8A, in accordance with embodiments. FIG. 9A is a flow diagram illustrating methods 901, 902 for recessing logic gate electrodes selectively to analog gate electrodes, in accordance with embodiments. It is noted that while the exemplary methods are described in furtherance of the exemplary embodiment where gate electrode z-height is differentiated between logic and analog gates, it will be appreciated that the exemplary methods are equally applicable to differentiating gate electrode z-heights for any subset of gate electrodes based on any circuit design criteria, or transistor function.

Referring first to FIG. 9A, the method 901 generally entails forming first and second gate electrodes (e.g., a logic and an analog gate) at given z-heights and then selectively etching back the first (logic) gate electrode relative to the second (analog) gate electrode to arrive at electrodes with differing z-heights. The exemplary method 901 begins at operation 905 with forming logic and analog gate electrodes having bottom electrode surfaces proximate to a gate dielectric disposed over semiconductor channel regions and having a top electrode surface at a nominal height from the bottom gate electrode surface. While operation 905 may be performed with any techniques known in the art and embodiments of the present invention are not limited in this respect, in the exemplary embodiment illustrated in FIGS. 2A and 2B, operation 905 entails first processing the substrate 105 to form trenches 260A and 260C exposing semiconductor channel regions 140 and 240. As further illustrated in FIGS. 3A and 3B, operation 905 then entails filling the trenches 260A and 260C with the gate dielectric 165, the work function materials 120A, 220A and the bulk materials 120B, 220B and polishing back any overburden to arrive at substantially planarized logic and analog gate electrode top surfaces of a substantially same z-height for both a logic gate and an analog gate.

The method 901 then proceeds to operation 925 where a protective mask is formed over an analog gate electrode as a basis for selectively recessing at least a portion of an unmasked (logic) gate electrode to a level below at least a portion of the protected top electrode surface. Formation of the protective mask may be non-trivial in view of the materials employed for the gate electrodes and their reactivity with mask materials and/or the susceptibility of the mask material to processes employed to recess one or more portions of the first gate electrode. For example, where a bulk material of the gate electrodes is tungsten, a tungsten etch process may have insufficient selectivity to a photosensitive mask (e.g., photo resist) or a subsequent photo resist strip (e.g., ash) may be detrimental to one or more materials present after recessing a gate electrode. As such, method 902 (FIG. 9B) illustrates advantageous embodiments where forming the protective mask further entails forming a multi-layered mask. In embodiments, the multi-layered mask comprises a plurality of layers, at least one of which is a non-photosensitive hard mask. In particularly advantageous embodiments where a gate electrode includes at least a metal bulk material, such as tungsten, the multi-layered mask includes a metal mask layer, formed in direct contact with the logic and analog gate electrodes at operation 930. In exemplary embodiments, the metal mask layer is a metal present in the gate electrode, for example in the bulk material or in a work function material.

FIGS. 4A and 4B further illustrate a metal mask layer 445 deposited over the substrate 105, in contact with top surfaces of the gate electrode bulk materials 120B, 220B, as well as the work function materials 120A, 220A. For the exemplary metal gate embodiments where the gate electrode bulk materials 120B, 220B and work function materials 120A, 220A are all metals, the metal mask 445 includes a metal which may be etched selectively to at least the metal(s) employed in the bulk materials 120B, 220B. In one exemplary embodiment where the bulk materials 120B, 220B are tungsten, the metal mask 445 comprises titanium (Ti) or a non-tungsten alloy thereof having good etching properties (e.g., TiN, TiAl, etc.). In advantageous embodiments, the metal mask 445 is a binary non-tungsten Ti alloy, which has only trace levels of impurities that are immaterial to the functions of the metal mask 445 described herein. As described further herein, depending on the embodiment, the metal mask 445 may have a range of thicknesses between 20 nm and 300 nm, for example.

Returning to FIG. 9B, with the metal mask layer in place, formation of the protective mask proceeds with operation 935 wherein a secondary mask is formed over a portion the metal mask layer that is disposed over only the analog gate electrode (i.e., the electrode that is to remain at a full z-height as formed at operation 905 (FIG. 9A). As illustrated by the dashed lines in FIG. 9B, the method 902 depends on whether the secondary mask formed at operation 935 is a photosensitive or non-photosensitive mask.

In one embodiment, where a photosensitive secondary mask (i.e., photo resist) is formed on the metal mask layer at operation 935, the method 902 proceeds to operation 940 where a partial thickness of the metal mask layer disposed over the logic gate electrode unprotected by the photo resist mask is etched. Any etch process known in the art to be suitable for the particular composition of the metal mask layer may be utilized at operation 940 with a goal being to controllably thin, but not clear, the metal mask layer in the regions which are to have recessed gate electrodes. For example, a timed etch performed operation 940 may provide a differential hard mask thickness over logic and analog gates which is to be subsequently utilized to expose the logic gate electrode. The method 902 then proceeds to operation 942 where the photo resist mask is removed by any conventional technique (e.g., plasma ash, wet strip, etc.). At operation 945 a blanket etch of the metal mask layer is then performed to clear the thinner portions of the metal mask layer (e.g., disposed over a logic gate) while only a partial thickness of the thicker regions of the metal mask layer is removed by the etch. For such embodiments, the metal mask layer is in the upper end of the range described elsewhere herein (e.g., 200 nm-300 nm) with the etch process being any wet or dry etch known to be suitable for the given composition of the metal mask layer (e.g., Ti alloy, etc.). With the metal mask layer now patterned as the protective mask, the method 902 returns to operation 946 in method 901 (FIG. 9A).

Continuing with description of FIG. 9B, where the secondary mask is non-photosensitive and one or more hard mask layer is deposited on the metal mask layer at operation 935, the method 902 proceeds to operation 943 where the portion of the hard mask disposed over the logic gate electrode is etched through to expose the metal mask layer. FIGS. 4A, 4B further illustrate one such embodiment where a hard mask layer 446 is deposited over the metal mask layer 445. The composition of the hard mask layer 446 is dependent on the composition of the metal mask layer 445 at least to the extent that a selective etch of the hard mask layer 446 is advantageous as is an etch process that can be performed over a logic gate selectively to an analog gate. While silicon dioxide and silicon nitride hard mask materials may be used in certain embodiments, in an exemplary embodiment where the metal mask layer 445 is a Ti alloy, the hard mask layer 445 is predominantly silicon. A silicon hard mask layer 445 can, under certain circumstances, be advantageously etched with good selectivity to TiN, and other Ti alloys. Over one portion of the hard mask layer, a photo resist pattern is then formed. In the exemplary embodiment further illustrated in FIG. 4B, a photo resist mask 447 is formed over a portion of the hard mask layer 445 disposed over the logic gate electrode (i.e., the gate that is to be recessed).

In embodiments, the material composition of one of the first and second portions the secondary mask is modified to enhance selectivity of an etchant employed in the etching of the secondary mask. Generally, modifying material composition of a hard mask layer forming the secondary mask may entail one or more of: oxidation, nitridation, or doping with a species, that either enhances or retards the etch rate of the modified portion of the material. FIGS. 4A and 4B further illustrate an exemplary embodiment where a dopant species is implanted into a portion of the hard mask 446 not protected by the photo resist mask 447. In the context of a silicon hard mask layer 446, doping of the silicon can serve as a basis of etch selectivity between doped and undoped regions for certain wet etch chemistries known in the art. For example, where the species is p-type (e.g., boron), the wet etch rate can be reduced relative to an undoped region. With the hard mask layer 446 composition selectively modified based on the photo resist pattern, the method 902 then proceeds to operation 942 where the photo resist mask is removed by any conventional technique (e.g., plasma ash, wet strip, etc.) such that photo resist selectivity of the process subsequently employed to etch the hard mask is inconsequential. The hard mask layer 446 is then blanket etched for a time sufficient to clear either the modified portion without clearing the unmodified portion (as for a silicon hard mask modified into p-type for the embodiment depicted in FIG. 5A, 5B), or vice versa. At operation 945 the metal mask layer unprotected (e.g., region over a logic gate) by the hard mask is etched to clear. For such embodiments, the metal mask layer may be advantageously relative thin (e.g., 20 nm-30 nm) and any wet or plasma-based metal etch process may be utilized depending on the metal mask composition. With the metal mask layer now patterned as the protective mask, the method 902 returns to operation 946 in method 901 (FIG. 9A).

In another embodiment of method 902 (FIG. 9B) where operation 935 entails depositing a hard mask, a multi-layered hard mask is deposited over the metal mask layer, for example to form a tri-layer mask. In a tri-layer mask embodiment, the hard mask layers deposited over the metal mask layer may be of any material and thickness that offer sufficient etch selectivity between each other if the layer in contact with the metal mask layer also offers sufficient etch selectivity relative to the metal mask layer. In one exemplary embodiment where a first hard mask layer in contact with the metal mask layer is silicon, the second hard mask layer is silicon dioxide (SiO₂), although other combinations may also be utilized. Following deposition of the multi-layered hard mask, a photo resist is patterned, and at least the first layer of the multi-layered hard mask is etched (e.g., silicon dioxide layer) using any technique conventional to the material. The method 902 then proceeds to operation 942 where the photo resist mask is removed by any conventional technique (e.g., plasma ash, wet strip, etc.) such that photo resist selectivity of the process employed to further etch the hard mask is inconsequential. After the second layer of the multi-layered hard mask is etched, the metal mask layer unprotected by the hard mask (e.g., region over a logic gate) is etched to clear at operation 945. For such embodiments, the metal mask layer may be advantageously relative thin (e.g., 20 nm-30 nm) and any wet or plasma-based metal etch process may be utilized depending on the metal mask composition. With the metal mask layer now patterned as the protective mask, the method 902 returns to operation 946 in method 901 (FIG. 9A).

As illustrated in FIG. 9A, at operation 946 the logic gate is etched back. As further illustrated in FIGS. 6A, 6B, gate electrode recesses 620 are first formed by etching the work function material 120A. In advantageous embodiments wherein the metal mask layer 445 includes a metal present in work function material, etching of the metal mask layer 445 may also serve to form the recesses 620. In other embodiments, the recesses 620 are formed with a second etch process, distinct from that used to pattern the metal mask layer 445. As further illustrated in FIG. 6A, 6B, the etchback of gate electrode further entails recessing the bulk material 120B. In advantageous embodiments, recessing of the bulk material 120B, for example with a plasma etch process where the bulk material 120B is tungsten, also etches through the hard mask layer 446 without etching through the metal mask layer 445, thereby maintaining protection of the bulk material 220B from the bulk recess etch while preparing for subsequent removal of the remain metal mask layer 445, if desired.

In embodiments, recessing of a gate electrode further comprises a second recessing of the work function material after recessing the bulk material. As illustrated in FIG. 7B recesses 720A are formed by a second etch of the work function material 120A that may, for example, occur during the removal of the metal mask 445 (e.g., at operation 965 in FIG. 9A), just as the first recessing of the work function material may occur during patterning of the metal mask 445. In further embodiments, as also illustrated in FIGS. 6B, 7B, removal of the metal mask 445 also recesses the work function material 220A relative to the bulk material 220B and/or the gate dielectric, forming recesses 720B. With the metal mask layer 445 being of a same or similar material as the work function material 120A, compatibility with the bulk material 120B with respect to etch chemistry and reactivity is advantageously ensured, and the gate recess etch process is efficient with little overhead associated with hard mask removal.

With the gate electrode z-heights now differentiated, the method 901 may proceed to operation 985 with completion of the logic and analog finFETs. For example, with formation of the source-drain contact metallization 131A, 231A, as illustrated in FIGS. 8A, 8B. Any conventional interconnect process may then be utilized to complete fabrication of an IC with selectively recessed gate electrodes. Alternatively, before interconnect processing is commenced, a blanket electrode recessing may be performed to non-selectively reduce the z-height of all gate electrodes below that provided at operation 905 while retaining a z-height difference between gate electrodes (e.g., a logic gate may be recessed by 45% relative to an initial height while an analog gate, or other logic gate is recessed by 25%). In still other embodiments, one or more additional iterations of method 901 are performed to achieve more than the two z-heights illustrated in the exemplary embodiment. For example, selective thinning of the metal hard mask with a separate mask may be combined with one of the exemplary hard mask technique to selectively thin portions of the metal mask remaining after gates in a first region are recessed. Second regions where the metal mask are thinned would then be exposed before third regions allowing for gate electrodes in the second regions to be recessed a second, lesser, amount while the gates in the first region are recessed again to a greater amount.

FIG. 10 is an isometric view and schematic of a mobile computing platform 700 which employs an IC including extended drain non-planar MOSFET ESD circuitry, in accordance with embodiments of the present invention. The mobile computing platform 700 may be any portable device configured for each of electronic data display, electronic data processing, and wireless electronic data transmission. For example, mobile computing platform 700 may be any of a tablet, a smart phone, laptop computer, etc. and includes a display screen 705 which in the exemplary embodiment is a touchscreen (capacitive, inductive, resistive, etc.), a chip-level (SoC) or package-level integrated system 710, and a battery 713.

The integrated system 710 is further illustrated in the expanded view 720. In the exemplary embodiment, packaged device 777 includes at least one memory chip (e.g., RAM), and/or at least one processor chip (e.g., a multi-core microprocessor and/or graphics processor) employing transistors with gate electrodes of differing z-height. In one particular embodiment, the package device 777 is a microprocessor with a logic gate of a lesser z-height than an analog gate. The packaged device 777 is further coupled to the board, substrate, or interposer 760 along with, one or more of a power management integrated circuit (PMIC) 715, RF (wireless) integrated circuit (RFIC) 725 including a wideband RF (wireless) transmitter and/or receiver (e.g., including a digital baseband and an analog front end module further comprises a power amplifier on a transmit path and a low noise amplifier on a receive path), and a controller thereof 711.

Functionally, the PMIC 715 performs battery power regulation, DC-to-DC conversion, etc., and so has an input coupled to the battery 713 and with an output providing a current supply to all the other functional modules. As further illustrated, in the exemplary embodiment the RFIC 725 has an output coupled to an antenna to provide to implement any of a number of wireless standards or protocols, including but not limited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE 802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE, GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well as any other wireless protocols that are designated as 3G, 4G, 5G, and beyond. In alternative implementations, each of these board-level modules may be integrated onto separate ICs coupled to the package substrate of the packaged device 777 or within a single IC (SoC) coupled to the package substrate of the packaged device 777.

FIG. 11 is a functional block diagram of a computing device 1000 in accordance with one embodiment of the invention. The computing device 1000 may be found inside the platform 700, for example, and further includes a board 1002 hosting a number of components, such as but not limited to a processor 1004 (e.g., an applications processor) and at least one communication chip 1006. In embodiments, at least one of the processor 1004 and communication chip 1006 incorporate transistors with selectively recessed gate electrodes, as have been described in the context of certain exemplary embodiments elsewhere herein. The processor 1004 is physically and electrically coupled to the board 1002. The processor 1004 includes an integrated circuit die packaged within the processor 1004. The term “processor” may refer to any device or portion of a device that processes electronic data from registers and/or memory to transform that electronic data into other electronic data that may be stored in registers and/or memory.

In some implementations at least one communication chip 1006 is also physically and electrically coupled to the board 1002. In further implementations, the communication chip 1006 is part of the processor 1004. Depending on its applications, computing device 1000 may include other components that may or may not be physically and electrically coupled to the board 1002. These other components include, but are not limited to, volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flash memory, a graphics processor, a digital signal processor, a crypto processor, a chipset, an antenna, touchscreen display, touchscreen controller, battery, audio codec, video codec, power amplifier, global positioning system (GPS) device, compass, accelerometer, gyroscope, speaker, camera, and mass storage device (such as hard disk drive, solid state drive (SSD), compact disk (CD), digital versatile disk (DVD), and so forth).

At least one of the communication chips 1006 enables wireless communications for the transfer of data to and from the computing device 1000. The term “wireless” and its derivatives may be used to describe circuits, devices, systems, methods, techniques, communications channels, etc., that may communicate data through the use of modulated electromagnetic radiation through a non-solid medium. The term does not imply that the associated devices do not contain any wires, although in some embodiments they might not. The communication chip 1006 may implement any of a number of wireless standards or protocols, including but not limited to those described elsewhere herein. The computing device 1000 may include a plurality of communication chips 1006. For instance, a first communication chip 1006 may be dedicated to shorter range wireless communications such as Wi-Fi and Bluetooth and a second communication chip 1006 may be dedicated to longer range wireless communications such as GPS, EDGE, GPRS, CDMA, WiMAX, LTE, Ev-DO, and others.

It will be recognized that the invention is not limited to the embodiments so described, but can be practiced with modification and alteration without departing from the scope of the appended claims. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 

We claim:
 1. An integrated circuit structure, comprising: a first MOS transistor, comprising: a first gate dielectric of the first MOS transistor, the first gate dielectric over a first fin, the first fin having a top; a first gate electrode of the first MOS transistor, the first gate electrode over the first gate dielectric, the first gate electrode having a first work function layer and a first bulk material, and the first gate electrode having a first gate length, wherein the first work function layer has a first height above the top of the first fin; a first source or drain structure adjacent a first side of the first gate electrode; and a second source or drain structure adjacent a second side of the first gate electrode, the second side opposite the first side; and a second MOS transistor, comprising: a second gate dielectric of the second MOS transistor, the second gate dielectric over a second fin, the second fin having a top; a second gate electrode of the second MOS transistor, the second gate electrode over the second gate dielectric, the second gate electrode having a second work function layer and a second bulk material, and the second gate electrode having a second gate length, the second gate length greater than the first gate length, wherein the second work function layer has a second height above the top of the second fin, the second height greater than the first height; a third source or drain structure adjacent a first side of the second gate electrode; and a fourth source or drain structure adjacent a second side of the second gate electrode, the second side opposite the first side.
 2. The integrated circuit structure of claim 1, further comprising: a first conductive contact structure in direct contact with the first source or drain structure, wherein the first bulk material of the first gate electrode has an uppermost surface below an uppermost surface of the first conductive contact structure; and a second conductive contact structure in direct contact with the third source or drain structure, wherein the second bulk material of the second gate electrode has an uppermost surface above an uppermost surface of the second conductive contact structure.
 3. The integrated circuit structure of claim 1, wherein the first gate dielectric has an uppermost surface above an uppermost surface of the first work function layer.
 4. The integrated circuit structure of claim 1, wherein an uppermost surface of the first bulk material is above an uppermost surface of the first work function layer.
 5. The integrated circuit structure of claim 1, wherein an uppermost surface of the first bulk material is below an uppermost surface of the first gate dielectric.
 6. The integrated circuit structure of claim 1, wherein the second gate dielectric has an uppermost surface above an uppermost surface of the second work function layer.
 7. The integrated circuit structure of claim 1, wherein an uppermost surface of the second bulk material is above an uppermost surface of the second work function layer.
 8. The integrated circuit structure of claim 1, wherein an uppermost surface of the first bulk material is above an uppermost surface of the first gate dielectric.
 9. The integrated circuit structure of claim 1, wherein the first MOS transistor is a logic transistor, and the second MOS transistor is an analog transistor.
 10. The integrated circuit structure of claim 9, wherein the logic transistor is an N-type logic transistor, and the analog transistor is an N-type analog transistor. 